Rare-earth-doped fiber amplifiers have been increasingly deployed in fiber optic signal transmission systems, with Raman-based amplification systems being tested for commercial deployment. Previously, over long distance fiber optic transmission links, the optical signal was detected at periodic distances by an opto-electronic detector and converted into an electrical signal, which was then used to drive a laser diode to in effect regenerate the optical signal for retransmission over the next section of the link. The distance between these opto-electronic systems was dictated by the attenuation at the signal frequencies of the fiber, and if any one of these opto-electronic devices failed, the entire optical transmission link failed. In contrast, fiber amplification systems enable optical signals to be re-amplified without conversion to electrical signals. With the advent of fiber amplification systems, the distance between electro-optical devices was no longer attenuation-limited.
Under the model, the fiber amplifiers are distributed along the link to amplify the optical signals or counter pumping into regular fiber is implemented with Raman pump arrays at the link terminus. Opto-electronic devices are only provided along the link at distances beyond which chromatic dispersion and other effects would impair signal demodulation.
Another advantage associated with the use of fiber amplification in optical transmission links is related to their broad gain spectrum. This feature makes dense wavelength division multiplexed systems realistic since multiple channels can be simultaneously amplified using a single fiber amplifier. Currently, WDM systems using fiber amplification have been deployed with 50-100 channels, with even larger channel systems being proposed.
The fiber amplification systems require relatively few components. They comprise a rare-earth-doped fiber or possible standard fiber in the case of Raman systems. Commonly, erbium-doped fiber amplifiers are used since they have a gain spectrum surrounding 1550 nanometers (nm), where there is a transmission window in commonly deployed silica optic fiber. Erbium-doped fiber amplification systems are usually pumped by laser diode pumps, operating at 980 nm or 1480 nm, while Raman systems use a broad range of pump wavelengths, ranging from about 1300 nm to 1600 nm.
Clean signal amplification is achieved by closely regulating the pump light. In broad-bandwidth WDM amplifiers, both power and wavelength stability of pump lasers should satisfy stringent requirements to avoid noise insertion into the signal frequencies. Additionally, shifting of pump wavelength can ruin gain flatness of fiber amplifiers. This robs power away from some channels and degrades their bit error rates. Typically, the wavelength of pump lasers increases slightly with increasing laser drive current as a fundamental result of the increase in junction temperature. The gain peak of semiconductor lasers typically shifts about 0.3 nm/xc2x0 C. These wavelength changes can induce changes in the gain spectrum of fiber amplifiers. See K. W. Bennet, et al, xe2x80x9c980 nm band pump wavelength tuning of the gain spectrum of EDFAsxe2x80x9d, OSA Annual Mtg., PD4-1, 1997. If the laser wavelength is controlled mainly by the semiconductor gain spectrum, wavelength control can be inadequate.
In some implementations spectral shifting of pump lasers is controlled by the use of fiber Bragg gratings between the laser pump chip and the fiber amplifier. These have the effect of helping to lock the emission wavelengths of the pump laser system. These fiber gratings, however, are expensive and complicate the deployment of the pump laser modules in amplifier systems.
Other methods have been used in the past to control the wavelength of semiconductor lasers. The earliest etalon stabilization techniques used an angled etalon, which serves as a transmissive filter. A reflection was provided by a mirror (or mirrors) on the opposite side of the etalon. This method, however, is not capable of providing a compact, integrated structure. See Berg, U.S. Pat. No. 4,081,760; Danielmeyer, U.S. Pat. No. 3,628,173.
External reflective mirrors or gratings (with filter and mirror integrated) have been used to feed a portion of the laser light back into the laser. External reflectors or gratings can induce instabilities in the output power due to multi-cavity effects. Abrupt mode hopping and larger power fluctuation often are induced due to cavity mode competition interacting with semiconductor laser nonlinearity. This in addition to the added complexity, positional sensitivity, temperature sensitivity, nonlinear power-current curves, and higher cost make this method generally inadequate for pump lasers.
Another configuration called the cleaved-coupled cavity (C3) laser uses a cleave or etched groove to create a second resonant cavity. It is difficult to apply protective coating to the internal facets of the cleaved cavity. As a result, oxidation of the internal cleaved surfaces causes deterioration of the laser. Moreover, the spacing between reflective surfaces is hard to control within a fraction of a percent. The C3 laser also has the added complication of a third electrode which must be driven to avoid semiconductor loss. See Allen, et al., U.S. Pat. No. 4,284,963; Scifres, et al., U.S. Pat. No. 4,358,851; Craig, et al., U.S. Pat. No. 5,185,754.
Another method uses a grating (or corrugation) within the semiconductor to achieve narrow band selectivity. This has been done with a distributed Bragg reflector (DBR) grating, which reflects a narrow band of light at one end of the cavity, or with a distributed feedback (DFB) grating, which fills the length of the laser. These both add significant complexity to the chip fabrication. Neither has shown adequate reliability and mode control to date at the powers needed for pump lasers.
Still another method uses an etalon that is selected to transmit only light in a desired narrow frequency band. Thin xcex/2 layers are used as spacers between multi-layer dielectric or single-layer metallic stacks. This is an example of a resonant etalon. The etalon is designed for maximum transmission at the desired wavelength. This geometry of etalon is also unable to provide subsequent trimming of the wavelength and it presents very low selectivity of the wavelength and has unproven ability to hit the desired design wavelength. See Jansen, et. al., U.S. Pat. No. 5,629,954.
Another structure has used dielectric coatings on an adjacent body attached with a resilient material to achieve single mode operation. See Smith, et al., U.S. Pat. No. 4,805,185. There are additional single mode lasers that use reflective structures that incorporate a spatially periodic structure adjacent to the laser. See Miller, et al, U.S. Pat. No. 4,675,873.
For pump lasers, single mode operation is not generally required; wavelength stabilization within a narrow band is desired. The previously described techniques do not provide for a wavelength selective element integrated directly on the laser. An integrated wavelength control element is preferable since it does not induce the additional closely spaced cavity modes that an external cavity does. Over the past few years, coating processes have improved significantly, making adequate low-stress, thick-film, reliable deposition possible directly on the semiconductor
Historically, the term etalon has referred to a Fabry-Perot etalon or interferometer. It includes a plane-parallel plate of thickness L2 and index ne which is bounded on each side by a partial reflector. See A. Yariv, Optical Electronics, 3rd Edition, Holt, Rinehart, and Winston Inc., chapter 4, 1985 and L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Circuits, John Wiley and Sons, Inc, pp 73-85, 1995. An etalon is a resonator.
More generally, etalon devices have been commonly used to restrict the longitudinal mode of operation. These intra-cavity structures typically have periodic spectral transmission peaks. These transmission peaks favor a few longitudinal modes within the laser""s gain curve, thus, yielding a narrower spectral envelope of longitudinal modes in the typical implementation. As previously mentioned, the lasing wavelength of a diode laser will shift about 0.3 nm/C when its wavelength is set by the material gain spectrum. It is preferable to have a less temperature-sensitive structure control the lasing wavelength. The integrated etalon structure can fulfill this need for more stringent wavelength stabilization requirements.
In semiconductor lasers, etalons have been demonstrated. For use in commercial systems, however, some form of integrated etalon would be required for manufacturability. Proposed designs, however, do not appear to have met with any success.
The present invention utilizes a system to spectrally select a narrow band of laser cavity modes. Rather than an intra-cavity device, however, the present invention utilizes an integrated, wavelength-selective facet reflector. This reflector functions essentially as a bandpass filter, which restricts the laser to only a narrow wavelength band. The reflector can be termed an anti-resonant etalon.
In general, according to one aspect, the invention features a semiconductor pump laser. This laser comprises a ridge waveguide electro-optical structure, which converts a ridge injection current into light. This is a common feature of most semiconductor lasers where a ridge is etched into at least a top cladding layer to form a waveguide. In some implementations, however, the ridge can be etched down through the active layer.
The pump laser further comprises an integrated wavelength selective facet reflector. This reflector controls the longitudinal modal operation of the pump laser. Specifically, the reflector comprises a first reflective structure for reflecting light to return through the ridge waveguide electro-optical structure. A second reflective structure provides wavelength-selective reflectivity when operating in combination with the first reflective structure. In other words, the phase of light reflected from the first and second reflective structures is such that the net reflectivity of the facet is wavelength selective, or favors certain wavelengths over other wavelengths.
In the typical implementation under current technology, the pump laser operates at approximately 980 nm and functions as a pump for a dense wavelength division multiplex (DWDM) system, utilizing erbium-doped fiber amplifiers. The wavelength selective reflectivity of the reflective structures provides modal stability and thus reduces temporal power fluctuations to reduce spectral gain shifts in the system that would be otherwise created by the pump laser. It should be noted, however, that the principles of the present invention can also be applied to other lasers to reduce power fluctations, such as those operating at 1480 nm for EDFA""s or the broader range of pump wavelengths used in Raman amplification schemes.
In the preferred embodiment, the total thickness on the first and second reflective structures is less than 100 micrometers. This limits light loss to beam divergence at the facet. In the preferred embodiment, however, it is substantially less, less than 20 micrometers.
In an implementation of a back etalon stabilized pump, the first reflective structure comprises about five quarter-wave layers of alternating low and high index material. The second reflector then comprises a relatively thick layer. This layer can have an optical thickness of 10-50, or specifically 30 wavelengths at the laser""s wavelength of operation.
When used as a pump laser, high power output, in addition to modal stability, is often desirable. As a result, some front facet reflectivity is preferred, typically between 3 and 10%. In the preferred embodiment of the back etalon stabilized pump, a front facet reflectivity is nominally 4%, specifically, 4% +/xe2x88x920.5%.